Anyway, it's a "shopping account" people sign up for. So, when you are making an online purchase at a participating online retailer, you sign in using your Google Checkout account, rather than having to set up an account with that store.
Which is great in a convenience soft of way, having to just remember one account's login and password. But of course I wonder about security and privacy.
I think this is very similar to what Microsoft was trying to do with Passport (now Windows Live ID?). So the idea really isn't new, but I do think people would be more willing to trust Google (with their financial information and purchase history) than Microsoft (but not me).
This is also similar to Library Elf's one-login-to-monitor-multiple-accounts idea, and Lichen has also talked about it before. I wonder, though, if Google, being as book-focused as they are, would be willing to open this program up to non-retail entities - like libraries.